Metal halide arc discharge lamps are frequently employed in commercial usage because of their luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within an outer jacket or envelope. The arc tube, itself hermetically sealed, has tungsten electrodes mounted therein and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
Double-ended metal halide lamps have been developed for low wattage and other special applications. An arc tube is mounted within a light-transmissive outer jacket and the ends of the outer jacket are press sealed, with the arc tube electrical leads extending through the press seals. The lamp is mechanically supported at both ends, and electrical energy is applied to opposite ends of the lamp. It has been found desirable to provide metal halide lamps with a shroud which comprises a generally cylindrical, light-transmissive member, such as quartz. The arc tube and the shroud are coaxially mounted within the outer jacket, with the arc tube located within the shroud. A double-ended metal halide arc discharge lamp utilizing a light-transmissive shroud is disclosed in U.S. application Ser. No. 07/866,381, filed Apr. 10, 1992 now U.S. Pat. No. 5,296,779.
Double-ended metal halide lamps typically have a power rating of 150 watts or lower. Existing lamps typically have an arc diameter to arc length ratio of about 0.73:1 and produce a color rendering index of 60 to 65 when dosed with a fill material including sodium iodide and scandium tri-iodide. Color rendering index (CRI) is a well-known measure of the ability of a lamp to reproduce color, with a CRI of 100 indicating exact representation of colors. The electric field strength in these lamps is typically about 50 volts per centimeter.
In some applications of metal halide lamps, careful optical control is required. Optical control is difficult with lamps which have a long arc length. Optical control is enhanced for lamps with relatively large arc diameter to arc length ratios, which more nearly approximate a point source. In such lamps however, the color rendering index, the luminous efficacy and the color temperature must meet or exceed the parameters obtained in prior art metal halide lamps.
It is a general object of the present invention to provide improved metal halide arc discharge lamps.
It is another object of the present invention to provide arc discharge lamps having a relatively large arc diameter to arc length ratio.
It is a further object of the present invention to provide arc discharge lamps which have a high color rendering index, a high luminous efficacy and a high color temperature.
It is yet another object of the present invention to provide metal halide arc discharge lamps which are low in cost and which are easily manufactured.